Chemical Vapour Deposition (CVD) of alumina on cutting tools has been an industrial practice for more than 15 years. The wear properties of Al2O3 as well as of other refractory materials have been discussed extensively in the literature.
The CVD-technique has also been used to produce coatings of other metal oxides, carbides and nitrides, the metal being selected from transition metals of the IVB, VB and VIB groups of the Periodic Table. Many of these compounds have found practical applications as wear resistant or protective coatings, but few have received as much attention as TiC, TiN and Al2O3.
Cemented carbide cutting tools coated with various types of Al2O3-coatings e.g., pure κ-Al2O3, mixtures of κ- and α-Al2O3 and very coarse-grained α-Al2O3 have been commercially available for many years. Al2O3 crystallizes in several different phases: α, κ, γ, β, θ, etc. The two most frequently occurring phases in CVD of wear resistant Al2O3-coatings are the thermodynamically stable, hexagonal α-phase and the metastable κ-phase. Generally, the κ-phase is fine-grained with a grain size in the range 0.5-2.0 μm and often exhibits a columnar coating morphology. Furthermore, κ-Al2O3 coatings are free from crystallographic defects and free from micropores or voids.
The α-Al2O3 grains are usually coarser with a grain size of 1-6 μm depending upon the deposition conditions. Porosity and crystallographic defects are in this case more common.
Often both α- and κ-phase are present in a CVD alumina coating deposited onto a cutting tool. In commercial cutting tools, Al2O3 is always applied on TiC coated carbide or ceramic substrates (see, e.g., U.S. Pat. No. 3,837,896, now U.S. Reissue Pat. No. 29,420) and therefore the interfacial chemical reactions between the TiC-surface and the alumina coating are of particular importance. In this context, the TiC layer should also be understood to include layers having the formula TiCxNyOz in which the carbon in TiC is completely or partly substituted by oxygen and/or nitrogen.
The practice of coating cemented carbide cutting tools with oxides to further increase their wear resistance is in itself well known as is evidenced in e.g., U.S. Reissue Pat. No. 29,420 and U.S. Pat. Nos. 4,399,168, 4,018,631, 4,490,191 and 4,463,033. These patents disclose oxide coated bodies and how different pretreatments e.g., of TiC-coated cemented carbide, enhance the adherence of the subsequently deposited oxide layer. Alumina coated bodies are further disclosed in U.S. Pat. Nos. 3,736,107, 5,071,696 and 5,137,774 wherein the Al2O3 layers comprise α, κ and/or α+κ combinations.
U.S. Pat. No. 4,619,866 to Smith describes a method for producing fast growing Al2O3 layers by utilizing a hydrolysis reaction of a metal halide under the influence of a dopant e.g., hydrogen sulphide (H2S) in the concentration range 0.01-0.2% at a CVD deposition temperature of 1000-1050° C. Under these process conditions, essentially two phases of Al2O3, the α and the κ phases, are produced. The resulting coating consists of a mixture of the smaller κ-grains and the larger α-grains. The process yields coatings with an even layer thickness distribution around the coated body.
Commonly owned Swedish Patent Application 9101953-9 (corresponding to U.S. patent application Ser. No. 07/902,721 filed Jun. 23, 1992, abandoned in favor of Ser. No. 08/238,341 filed May 5, 1994, the disclosures of which are hereby incorporated by reference) discloses a method of growing a fine-grained κ-alumina coating.
Commonly owned Swedish Patent Application No. 9203852-0 (corresponding to U.S. patent application Ser. No. 08/159,217 filed Nov. 30, 1993, the disclosure of which is hereby incorporated by reference) discloses a method for obtaining a fine-grained, (012)-textured α-Al2O3-coating. This particular Al2O3-coating applied on cemented carbide tools has been found particularly useful for machining of cast iron.
Commonly owned Swedish Patent Application No. 9304283-6 discloses a body with a coating comprising one or more refractory layers of which at least one layer is a layer of α-Al2O3 textured in the (110) direction. The alumina layer is essentially free of cooling cracks and comprises platelike grains with a length of 2-8 μm and a length/width-ratio of 1-10.